Praseodymium doped arsenic sulfide single mode fibers were prepared and investigated with regard to their suitability for 1.3 micrometers amplification. Although these fibers have relatively low basic losses of about 2 dB m-1 at pump (1.02micrometers ) and signal wavelength, till now no signal amplification could be measured because of a strong attenuation induced by the continuous wave (cw) pump light. In order to investigate the more detailed optical behavior, we have carried out time resolved measurements with short pump pulses of 1-5 ms at a repetition rate of 1 Hz on a 2 m fiber length. The evaluation of the time dependent signal intensity at the fiber end in comparison to the fluorescence intensity without signal has shown the fast development of a transient absorption within 0.5-1 s. Moreover, an inner gain could be clearly detected which increased with the pump power (0.75-75 mW). The gain coefficient related to the input pump intensity has been determined to be 0.5 dB- mW-1.